1. Field of the Invention
This invention relates to a light-emitting semiconductor device having enhanced brightness, particularly to one for enhancing current distribution of a front contact in a light emitting diode, so as to enhance the light emitting efficiency of a light-emitting semiconductor.
2. Background of the Invention
The principles lying behind luminance of light emitting diodes relate to passing current sequentially through P-N junctions of a semiconductor to generate light, wherein AlGaInP is implemented in high brightness red, orange, yellow and yellowish green LEDs, AlGaInN is in blue and green LEDs. The process of metal organic vapor phase epitaxy (MOVPE) is commonly adopted in the mass production of the LEDs, while the light-emitting components are of the structures, including: homo-junction (HOMO), single-heterostructure (SH), double-heterostructure (DH), single-quantum well (SQW) and multiple-quantum well (MQW) or other appropriate structures.
The structure of a conventional light emitting diode is illustrated in FIG. 1A, including, from the top down, a front contact 11, an active layer 12, a substrate 10 and a back contact 13. The active layer 12 is formed by a light-emitting material, such as AlGaInP or AlGaInN by adopting MOVPE. After current is injected through the front contact 11, the current will pass through the active layer 12 and the substrate 10 to flow towards the hack contact 13. Light is emitted when the current flows through the active layer 12. However, the low carrier mobility and high resistance of the active layer made of AlGaInP or AlGaInN results in poor electric conductivity of the AlGaInP or AlGaInN. When current is applied to the front contact located above the active layer 12, even if a capping layer 14 (or window layer) is added to enhance the current distribution to make minor improvements to the current distribution, the current is still concentrated at the lower portion of the contact such that the primary emitting regions are mainly concentrated at and next to the lower portion of the contact, as illustrated in FIG. 1B.
The refractive index (n=3.4˜3.5) of most materials for making semiconductor LEDs is greater than the surrounding refractive index (n=1˜1.5, n=1.5 for epoxy). In other words, a great portion of the light emitted by a semiconductor LED is completely reflected back to the semiconductor by the interface between the semiconductor and its exterior epoxy. The portion of the light that has been completely reflected is then absorbed by the active layer, the contacts and the substrate thereby reducing the actual luminance beneficial results of the LED (as shown in FIG. 1C).
To enhance the current distribution, improvements have been made to the structures and materials, such as that disclosed in U.S. Pat. No. 5,008,718 by Fletcher et al., where a capping layer 15 (or window layer), made of GaP, GaAsP and AlGaAs having a low resistance value and being pervious to light, is added between the front contact and active layer, as shown in FIG. 1D. The objective of using this capping layer is to enhance the current distribution flowing from the front contact. As described in the '718 patent, to improve the current distribution, the capping layer is preferred to be in the range from 150 to 200 micrometers thick to enhance the luminous intensity by 5 to 10 times. However, the increasing thickness of the capping layer also increases the time and cost required for MOVPE epitaxy thereby significantly increasing the cost of the epitaxy. In addition, the distribution ability is extremely relevant to the thickness. Hence, to ensure even current distribution, the thickness must be at least 10 micrometers or the current crowding problem cannot be effectively resolved.
Another measure is to change the design of contacts. F. A. Kish and R. M. Fletcher suggested re-designing the contacts to include fingers 16 (as shown in FIG. 1E) or extended with Mesh lines 17 (as shown in FIG. 1F), to resolve the current crowding problem in LEDs. The result, however, is not satisfactory because the inherent width of the Mesh lines extending from the contacts usually ranges from 5 to 25 micrometers to ensure easy production. The number of fingers or Mesh lines of such a width must be limited in order to prevent excessive masking of light, such that the light emitted below the contacts would all be masked by the fingers or Mesh lines. Since the current located exactly below the contacts are most intensive to result in intensive illumination, the metal meshes mask the regions that are intensively illuminated. However, reducing the number the metal meshes will cause poor current distribution at some of the luminous regions E so as to affect the light-emitting effects (as shown in FIGS. 1G to 1H).
To improve the current distribution, this invention discloses another design for the contacts so as to provide even current distribution and to reduce the regions masked by the contacts thereby enhancing the brightness.